The present invention relates to a surface emitting semiconductor laser device, a method for fabricating the same, and a surface-emitting semiconductor laser array that employs the laser devices.
A surface-emitting laser (device) that emits laser light in the direction normal to the plane of the substrate can be easily coupled to an optical fiber because of the circular shape of the emitted light beam. In addition, the surface-emitting laser can have an optical resonator which is short in length and which emits single mode light. Thus, the surface-emitting laser has lately received attention as a light source for use in data communications (optical interconnections) or optical computers, which employ optical fibers.
Furthermore, the surface-emitting laser device having a small region of lasing, i.e., an active layer, can be operated at a low threshold current (for example, on the order of several milliamperes). Furthermore, it is expected that an array of a large number of these laser devices will be applied as a highly integrated device.
An example of such a surface-emitting laser device is shown in FIG. 1.
The laser device comprises the following layers grown on a p-type GaAs semiconductor substrate 100 in the order mentioned below. That is, grown are a lower reflector layer structure 110 of a p-type AlGaAs multilayer film, a current confinement layer 120, a quantum-well-structured active layer 140 of GaAs/AlGaAs, and an upper reflector layer structure 150 of an n-type AlGaAs multilayer film. The upper portion from the upper reflector layer structure 150 to the lower surface of the current confinement layer 120 (the interface between the current confinement layer 120 and the lower reflector layer structure 110) is formed into a cylindrical mesa structure (of diameter 20 xcexcm) 200. On the upper end surface of the mesa structure, an n-type electrode 160 is so formed by evaporation as to cover the mesa structure 200. On the other hand, there is formed a p-type electrode 180 on the rear surface of the substrate 100. Incidentally, a silicon nitride film 190 is grown to passivate the surface of the device except for the upper end surface of the mesa structure, that is, the side of the mesa structure and the upper surface of the upper reflector layer structure.
To form the mesa structure 200, dry etching such as reactive ion beam etching (RIBE) is performed on the entire layer structure after the aforementioned layers have been grown on the substrate 100.
The current confinement layer 120 located between the lower reflector layer structure 110 and the active layer 140 is formed as follows: A precursor layer is first formed of, e.g., AlAs and then a mesa structure including the precursor layer is formed. Thereafter, the mesa structure is heated (for example, at 400xc2x0 C. for ten minutes) in a water vapor atmosphere. Thereby, the precursor layer is oxidized from the side portion toward the core portion thereof, to form an insulating layer 120a and allow a non-oxidized electrically conductive layer 120b of AlAs to remain on the core portion. In this case, injected current is concentrated to the electrically conductive layer 120b of the current confinement layer 120, thereby making it possible to reduce the threshold current of the device.
Incidentally, to form the current confinement layer 120 from the precursor layer in a stable state, it is necessary to allow the precursor layer to be positively included within the mesa structure and perform sufficient oxidation from the side portion. However, in a case where dry etching such as RIBE having directionality is employed to form the precursor layer in the mesa structure, instability of the dry etching causes the depth of an actual etching to be varied by approximately xc2x110%.
Therefore, to etch the whole layer structure in consideration of such a variation with the precursor layer being positively included therein, it is necessary to set the endpoint of the etching to a position deeper by 10% or more than the actual position of the precursor layer. For example, in a case where the precursor layer is present at a depth five xcexcm from the surface of the whole layer structure, the endpoint of the etching can be set to a position 5.5 xcexcm from the surface to provide an actual etching depth of approximately 5 to 6 micrometers. Thus, in any case, it is possible to form the mesa structure with the precursor layer being positively included therein.
However, with this method, if the depth of an actual etching is shifted to be larger than the value of the endpoint that has been set, the portion under the precursor layer is to be etched. For example, in the aforementioned example, the whole layer structure may be etched to a depth one xcexcm deeper at maximum than the position of the precursor layer as shown by the dashed line in FIG. 1. Thereby, part or all of the lower reflector layer structure 110 of p-type may be formed into a mesa structure.
Japanese Patent Laid-Open Publication No. Hei 5-235464 disclosed the following fact. As the cross-sectional area of a p-type GaAs/AlAs mirror layer (reflector structure) of a laser device decreases, a spike or difference in valence bands at the heterojunction of GaAs/AlAs tends to limit the electrical conduction of holes, thereby causing an increase in resistance of the device. Therefore, the surface-emitting semiconductor laser device employing a p-type semiconductor substrate presents a problem such that a p-type reflector layer structure on the substrate is etched into a mesa structure having a reduced cross-sectional area, and thus the device has a higher resistance than that of a device having the reflector layer structure being not etched, resulting in an increase in the heat generation and operating voltage of the device. Furthermore, a variation in depth of etching causes the resulting device to be varied in property.
An object of the present invention is to solve the aforementioned problems of the surface-emitting semiconductor laser device by providing a surface-emitting semiconductor laser device, a method for fabricating the device and a surface-emitting semiconductor laser array employing the device, in which the lower reflector layer structure is prevented from being etched by an etching blocking layer provided between the current confinement layer and the lower reflector layer structure and the resistance of the device is reduced.
To achieve the aforementioned object, the present invention provides a surface-emitting semiconductor laser device comprising a lower reflector layer structure and an upper reflector layer structure, formed on a p-type semiconductor substrate. The device also comprises an etching blocking layer, a current confinement layer, and an active layer, formed in that order from below between said lower reflector layer structure and said upper reflector layer structure. In the device, a portion over said etching blocking layer is formed into a mesa shape.
In addition, the present invention provides a method for fabricating the surface-emitting semiconductor laser device comprising the steps of forming a lower reflector layer structure, an etching blocking layer, a precursor layer of a current confinement layer, and an upper reflector layer structure, on a p-type semiconductor substrate in that order; performing dry etching from the upper reflector layer structure downwards to form a general shape of a mesa structure over the etching blocking layer; performing wet etching up to an upper surface of the etching blocking layer to form a shape of the mesa structure on the upper surface of the etching blocking layer; and oxidizing a side portion of the precursor layer of the current confinement layer included within the mesa structure, to form the current confinement layer. Furthermore, the present invention provides a surface-emitting semiconductor laser array comprising a plurality of layer structures each including from said lower reflector layer structure to said upper reflector layer structure of the above-mentioned surface-emitting semiconductor laser device, wherein the plurality of layer structures are integrated on a p-type semiconductor substrate that is common among them.